This invention relates to a method of molding a deformable fabric or thermoformable sheet, and in particular to a method of molding under vacuum and/or pressure a cold or hot deformable fluid-impervious fabric or a sheet of thermoformable plastic material in the cavity of a female mold for manufacturing flexible foam cushions of plastic material having an integrated fabric cover, and to a diaphragm for use in carrying out the method.
It is known to manufacture flexible foam cushions of plastic material having an integrated fabric cover by arranging a cold or hot deformable fabric, which has been made airtight (fluid-impervious) by an elastic coating applied to one of its faces, in a female mold, by fixing the cover to the periphery of the mold under vacuum, and by casting into the mold and onto the fabric cover arranged therein a foaming mixture capable of producing in situ a flexible foam of plastic material. After the foam has polymerized, the cushion together with its fabric cover adhering to the foam is removed from the mold.
A major difficulty of this known method is obtaining a uniform stretch and thickness in the fabric cover of the finished article. In fact, upon applying a vacuum to the mold, the fabric is not deformed uniformly as it undergoes stretching, because as the fabric cover contacts the shallower portions of the mold cavity, it is held there by the simultaneous action of vacuum and friction. In order to be able to also take the form of the deeper portions of the mold cavity, the fabric then has to stretch to a greater extent at the places where it is not adhering to the cavity wall. This results in the fabric being stretched unevenly and, therefore, in differences in the thickness of the cover of the finished cushion.
In order to obtain a uniform thickness throughout the cover fabric, it would be necessary for the fabric to be able to slide on the cavity wall in order to be uniformly distributed in the cavity. This is prevented or resisted, however, for the reasons referred to unless use is made of a liquid or powder lubricant capable of allowing the fabric to slide. Use of a lubricant under the conditions encountered is generally not possible without damaging the cover of the finished article.
To overcome this problem, it is possible to use one or more pistons which push the fabric towards the deep portions of the mold before applying the vacuum. Under the action of the pistons, the fabric as a whole then undergoes a first deformation, which is uniform to a greater or lesser extent, in order to give it a shape substantially similar to that of the mold cavity before applying the vacuum. The application of a vacuum then causes auxiliary fabric deformation to a relatively reduced extent against the inner wall throughout the mold.
This process which allows the fabric to be placed in a mold by causing the fabric as a whole to undergo a relatively uniform stretching requires, however, the use of auxiliary equipment which becomes quite complicated and expensive when the mold has a complicated pattern, and the system cannot practically be employed with moving molds such as those used in a cushion production line.